Start by preheating your oven to 350°F (175°C). This ensures that when the batter is ready, it can go directly into a hot oven, which is crucial for even baking.

Grease a round 9-inch cake pan. Using parchment paper at the bottom adds an extra layer of non-stick assurance. You don’t want your masterpiece to stick when you’re ready to serve.

In a medium saucepan, combine the butter and bittersweet chocolate. Melt them over low heat while stirring gently. This creates a luscious chocolate mixture that forms the foundation of your cake.

In a separate bowl, combine the granulated sugar and water. Microwave this mixture until the sugar is dissolved. This process helps integrate the sugar effortlessly into the chocolate mixture, ensuring no graininess in your cake.

Once the chocolate and butter have melted, remove from heat. Gradually whisk in the sugar-water mixture. This should be done slowly to avoid cooking the eggs in the next step.

In a separate bowl, crack the eggs and beat them lightly. Slowly add the eggs to the chocolate mixture, whisking constantly. This will help to avoid scrambling the eggs. Add in the vanilla extract, espresso powder, and sea salt.

Pour the batter into the prepared cake pan. Give it a gentle tap to remove any air bubbles.

Bake in the preheated oven for 25-30 minutes. The center should be set but slightly wobbly; it will firm up upon cooling.

Once baked, allow the cake to cool in the pan for about 10 minutes before inverting it onto a serving plate. Dust with cocoa powder or powdered sugar for that final touch.